A hybrid electric vehicle (HEV) has been developed to improve environmental protection and performance. The HEV includes two driving sources of an engine and a motor and an inverter which converts a DC current of a secondary battery into an AC current and supplies the AC current to the motor. The HEV requires an apparatus for cooling the motor and the inverter, in addition to the existing engine cooling system. Therefore, the existing HEV is provided with an engine cooling circuit for engine cooling and an HEV cooling circuit for an HEV system, and circulates cooling water to the cooling circuits to cool the engine or the HEV system. However, since a cooling water control temperature (85° C. to 95° C.) of the engine cooling circuit and a cooling water control temperature (35° C. to 50° C.) of the HEV cooling circuit are different, the engine cooling circuit and the HEV cooling circuit are independent from each other.
Therefore, according to the related art, when the HEV system is overheated due to lack of cooling water of the HEV cooling circuit, a failure of a HEV water pump, or the like, driving may be impossible.